This invention relates to a method and apparatus for drying paper-wound bushings of the type used to bring high voltage into high voltage electrical equipment, such as transformers and switchgear.
Bushings for high voltage equipment comprise a hollow tubular conductor surrounded by a plurality of layers of paper forming an insulation wrap about the central conductor. In conventional bushings the wrapped paper is impregnated with degassed oil to enhance the dielectric properties of the bushing, and a ceramic sheath is disposed about the paper wraps before installation of the bushing in an actual electrical system. During manufacture of the bushings, the paper is wrapped about the conductor and saturated with water to facilitate tight wrapping of the paper. Following manufacture it is necessary to dry the bushings to remove essentially all the water from the wrapped paper, since water reduces the electrical breakdown strength of the insulation.
Prior art attempts to dry high voltage bushings have included placing the bushings in a heated, evacuated tank and holding them until the bushings have been heated to a temperature adequate to drive all the water from the paper wraps. This has required time periods as long as three weeks for large bushings. The tanks are conventionally heated by such means as steam, oil or direct resistance heat being applied to the inner wall of the tank, while the pressure in the tank is reduced to a vacuum condition. Since the air was removed from the tank, the predominant, and, in fact, nearly the only, transmitter of heat from the tank wall to the bushings was radiation. This heating technique is slow and subject to the disadvantage that the bushings disposed near the tank wall interfere with the radiational heating of the radially-inner bushings, further extending the process time required to adequately dry the bushings. The long time requirement for drying the bushings is the limiting factor on the rate of bushing manufacture using prior art drying techniques.